Cassini team spies moonlets in Saturn's A ring

Many years ago, a comet strike or a wandering asteroid passing through Saturn's moon system, crashed into one of the orbiting bodies, shattering it and sending fragments the size of sports stadia whirling along its orbital path.

Moonlets orbiting Saturn. Credit: NASA

New images from NASA's Cassini spacecraft have identified a series of "propellor-like" features that researchers think are the result of such a collision. The Cassini imaging team, including experts from the University of Boulder, Colorado, interprets the propellers as being the wake caused by small moonlets.

These bodies are too small to clear out surrounding ring material, but can still attract smaller particles to form the propeller shape. The largest of those in the image (in the white edged box out) is said to be roughly 150 metres wide. The size is inferred from the separation of the propeller wings.

The first such features were seen in 2004, but the latest pictures have revealed that the moonlets are grouped together in a 3,000km wide belt of material.

"This is the first evidence of a moonlet belt in any of Saturn's rings," said Miodrag Sremcevic of CU-Boulder's Laboratory for atmospheric and space physics. "We have firmly established these moonlets exist in a relatively narrow region of the 'A' ring, and the evidence indicates they are remnants of a larger moon that was shattered by a meteoroid or comet."

The team calculates that the region may hold many thousands of these moonlets, ranging in size from a small truck to that of a sports stadium. Each propeller feature is about 16km long, the result of gravitational disturbances of the dusty ring.

The researchers say the findings support the idea that Saturn's rings were all created during a "collisional cascade" of ring debris, prompted by the breakup of a much larger moon. But the moonlets belt probably came later, after the ring system was well established (scientists think Saturn's rings have been there for hundreds of millions, possibly even billions of years).

In the paper, published in Nature, the team writes: "It seems unlikely that moonlets are remainders of a single catastrophic event that created the whole ring system, because in this case a uniform distribution would emerge. Instead, the moonlet belt is compatible with a more recent body orbiting in the A ring."